1. Field of the Invention
This invention relates to a method of converting triglycerides to biofuels. The triglycerides are derived from renewable sources, including vegetable oils. The biofuels include the grades of gasoline, jet fuel, diesel, and heavy oil.
2. Discussion of Related Art
Vegetable oils are triglycerides with the generic formula shown below for soybean oil (with triglycerides and unsaturated fatty acids shown):
where R1, R2, and R3 are the same or different hydrocarbon residues of fatty acids. Variations in crop oils contribute to different types and proportions of fatty acids in the triglycerides.
Crop oils that contain more than 30% polyunsaturated fatty acids (a preferred feedstock for the process of the present invention) include corn oil, cottonseed oil, linseed oil, peanut oil, safflower oil, soybean oil, sunflower oil and tung oil. The table below sets forth the various types of poly-unsaturated fatty acids present in these crop oils.
Composition of Selected Crop OilsFatty Acid, wt %Crop Oil16:018:018:118:218:3Polyunsat. Fat Type14:0Palmitic16:1StearicOleicLinoleicLinolenic20:022:0Corn oil12.20.12.227.557.00.90.1Linseed oil7.04.039.015.035.0Peanut oil0.111.60.23.146.531.41.53.0Soybean oil0.111.04.023.453.27.80.30.1Sunflower oil0.26.80.14.718.668.20.50.4Tung oil3.12.111.214.669.0**cis-9, trans-11, trans-13-octadecatrienoic acid
Soybean oil is 61% polyunsaturated. Over 80% of vegetable oil production in the United States is soybean oil. For these reasons, soybean oil may be the most preferred material for biofuel production using the process of the present invention. The soybean compositions include hulls (8wt %), oil (20wt %), protein (43wt %), ash (5 wt %), and water (balance).
The concept of converting vegetable oils into engine fuels was first attempted more than 100 years ago. During the late 1930s through 1940s, due to the shortage of petroleum, industrial-scale plants became operational to produce gasoline, kerosene, and other grades of fuel from vegetable oils. By the nature of the molecular structures of vegetable oils, heat is required for any fuel conversion processes. According to a recent review, various processes for converting crop oils into biofuels can be summarized into three categories as shown in FIG. 1. Among these conventional processes that are typically carried out at near atmospheric pressures, direct pyrolysis or catalytic cracking is the most common approach. Converting soybean oil into fatty acid salt (soap) followed by catalytic cracking is also reported. Recently, biodiesel production from transesterification of soybean oil has become popular. Biodiesel can also be further processed using conventional upgrading and post refining processes that typically include one or more of the following unit operations: catalytic cracking, isomerization, aromatization, and hydrogenation. However, each of these processes require high temperatures, which in addition to being costly, also causes oil degradation, resulting in loss of valuable oil to gaseous byproducts and less valuable coke.
High-pressure processes for both hydrolysis and transesterification of soybean oil have been recently demonstrated as shown in FIG. 2. These processes proceed more rapidly than the low-pressure processes and do not require catalysts. Pressure also appears to enhance direct pyrolysis and/or catalytic cracking of vegetable oils. It has been reported that a yield of 75% “crude oil” can be obtained by cracking tung oil at pressures ranging from 3-10 bar (50-150 psig) and at temperatures from 400-500° C. Based on boiling point, the resulting “crude oil” consisted of 50% gasoline, 30% kerosene, and 20% fuel oils. Therefore, high-pressure processes hold the potential for successfully converting crop oils into biofuel surrogate with relatively short reaction time, yet high mass and energy conversion efficiencies.